This invention relates generally to magnetic disc drive data storage devices and more particularly to an improved method for the accurate detection of the physical crossing of a transducer across cylinders of information recorded thereon.
A disc drive typically comprises a disc pack consisting of a plurality of magnetic recording discs each having a multiplicity of concentric recording tracks and being mounted on a drive spindle in stacked, slightly spaced relation to one another for common rotation about the spindle. The disc drive further comprises an array of magnetic heads disposed in read/write relation with the discs. In the case of moveable head drives, the heads are mounted on an actuator-driven carriage mechanism with at least one head operatively associated with each magnetic surface. In such drives the heads are usually moved substantially radially across the discs to access a desired track on any disc.
Information is stored on a surface of a disc by being recorded thereon in a serial format in concentric rings or tracks known in the art as cylinders. The concentric cylinders are arranged across the surface of a disc with varing radaii. The density at which cylinders are placed on a disc vary greatly, from sixty tracks per inch to over one thousand tracks per inch.
In either placing information on the disc or reading it therefrom, the magnetic head associated with the surface of the disc having the cylinder of interest is moved radially either in or out across the surface of the disc to position the head over the correct cylinder having the desired information. As information is stored on the surface of magnetic discs in cylinders it becomes necessary to be able to locate desired cylinders for the reading or writing of information thereon. Consequently a signal related to the radial motion of the carriage mechanism is necessary to properly locate the carriage mechanism carrying the heads over the desired cylinder.
In some prior art disc drives, a signal related to the radial motion of the carriage mechanism is obtained by means of apparatus which includes an optical grating mounted for movement with the carriage mechanism and operating in combination with a light source and associated light detector fixedly positioned to each side of the grating. As the grating moves with the carriage mechanism, the light passing through the grating is modulated and the distance traveled by the heads determined by detecting the modulated light, thereby providing an accurate indication of head position relative to the tracks on the discs. An example of such a head positioning system is disclosed in U.S. Pat. No. 3,597,750, entitled Servo with AGC for Positioning a Magnetic Head, issued Aug. 3, 1971.
In newer disc drives the optical grating has been replaced by the use of position control servo data which is recorded directly on the surface of one of the magnetic discs. Such system typically includes a servo surface prerecorded on one disc surface in a disc pack and a servo head which cooperates with the servo surface and is mounted for movement in unison with the read-write heads associated with the disc data surfaces. The servo surface is prerecorded with a plurality of concentric magnetic tracks, and when the servo head moves radially across the tracks, the signal induced in the servo head varies between a maximum value when the servo head is in direct alignment with a track and a minimum value when the servo head is midway between two adjacent tracks. The servo head signal is demodulated to form a head position signal. In accessing a desired servo track, the moveable member containing the magnetic heads is then moved across many tracks, each of which is counted, until the desired track is reached.
In such a servo system, the waveform which results from the servo head as it moves over the concentric prerecorded magnetic tracks on the servo surface accordingly varies between minimum and maximum values as each track is crossed. By detecting and counting these changes, the location of the magnetic record/reproduce heads relative to the cylinders is known.
Typically the sensing of the waveform from the servo head is performed by use of level detectors with analog hysteresis. A common implementation of such a device includes comparators and associated hysteresis circuits. In such a circuit the threshold amplitudes depend strongly on the normal peak amplitude of the input signal, and consequently requires that the peak amplitude of the input signal be maintained very constant. This is essential for accurate level detection.
To achieve constant peak amplitude for the input signal at different linear velocities of the servo transducer, rather sophisticated automatic gain control and velocity controlled demodulation circuits have to be used in generating the signal to be supplied to the comparators and associated hysteresis circuits. Consequently prior art implementations employing such circuits are quite sensitive to the peak amplitude of the input signal, and require substantial efforts in the area of analog signal processing to maintain the desired constant level.
Furthermore, due to the properties of the process of magnetic recording and the demands of high track densities, the normal peak amplitude of the signal from the servo head is rounded off before reaching the ideal peak amplitude. This phenomenon has undesirable results when it is used with a circuit employing hysteresis, as it results in a reduction of the useable difference between the maximum and minimum values of the waveform from the servo data. This in turn results in the reduction of the amount of useable hysteresis in the circuit, and hence a reduction in the safety margin for noise.
In addition to the above described problems, circuits employing comparators with associated hysteresis circuits are very susceptible to noise on the input signals thereto with amplitudes which exceed the hysteresis switching levels. The occurrence of such transients on the input to such circuits could, depending upon the magnitude thereof and the associated hysteresis level, result in the switching of the circuit and the consequent generation of a false output. This in turn would result in an error in the count of the servo cylinders.